Triboelectric coating powder and process for coating wood substrates

ABSTRACT

An electrically chargeable dielectric coating powder and a triboelectric coating process for applying the powder to wood substrates are described. The coating powder is a mass of finely divided, heat fusible dielectric plastic material having an average particle size (Mv) of between 30 and 45 microns and a particle size distribution (all percents defined in weight percent) of: 
     95%-100% smaller than 88 microns, 
     5%-15% smaller than 15.56 microns and 
     0%-6% smaller than 11 microns. 
     Preferably the coating powder has 0% larger than 88 microns and an Mv=about 30-40, preferably 35-40 microns. More preferably the particle distribution further includes 
     10%-15% smaller than 15.56 microns and 
     4%-6% smaller than 11 microns, and 
     an Mv of between about 35 and 36 microns. Most preferably the particle distribution further includes: 
     about 11.5% smaller than 15.56 microns and 
     about 4.3% smaller than 11 microns, and 
     an Mv of about 35.9. Preferably the powder is a thermosetting resin composition.

CROSS-REFERENCES

This is a divisional of application Ser. No. 08/644,553 filed on May 10,1996 now U.S. Pat. No. 5,731,042, which is a continuation-in-part ofapplication Ser. No. 08/169,793 filed on Dec. 20, 1993, now U.S. Pat.No. 5,552,191 which in turn is a continuation of application Ser. No.07/837,459, filed Feb. 14, 1992, now abandoned.

FIELD OF THE INVENTION

This invention relates to an electrostatically chargeable dielectriccoating powder and a triboelectric coating process for application ofthe powder to wood substrates. More particularly the invention pertainsto a mass of finely divided, heat fusible, insulating plastic or resinmaterial having a particular average particle size and a particularparticle size distribution, and a triboelectric coating process forcoating wood substrates utilizing this powdered material whereby powderoverspray and reclamation are minimized. This invention, in particularrelates to the triboelectric coating of wood with a coating powder andlow temperature curing which results in a continuous decorative orprotective coating having a minimum number of pinholes.

DESCRIPTION OF THE PRIOR ART

A number of solventless-type painting or coating systems have beendeveloped in which a finely divided, heat fusible material is depositedon a substrate, which deposit is then fused into a continuous functionalor decorative film on the substrate. Representative of these typeprocesses are flame spraying, fluidized bed, hot flocking, electrostaticspray (ESP) and electrostatic fluidized bed (ESFB). ESFB is a hybrid offluidized bed and ESP, as explained, for example, in U.S. Pat. No.4,689,241.

Flame spraying and fluidized bed coating processes typically applythermoplastic resin powders as coatings on metal or other substrates.These techniques, however, proved to have several major disadvantages,one being a general inability to coat at a film thickness much belowabout 250 microns (10 mils), and another being the necessity to have onhand a relatively large reservoir of each powder used. Subsequentlythermosetting powders, for example, epoxides, were utilized and found tobe superior to thermoplastic resin powders first used in ESP processes.The inherent advantages of thermosetting resin systems overthermoplastics in powder coating, especially ESP, are well known.

Continual development in the ESP technology provided the breakthroughnecessary to make powder coating an economical alternative toconventional liquid coating. In the typical ESP process the coatingpowder is maintained in a fluidized bed reservoir, injected into an airstream and carried to a spray gun where the powder is charged by passingthrough a stable corona discharge created by a high voltage source. Thecharged powder is then transported to a grounded part or substrate to becoated through a combination of electrostatic and aerodynamic forces.The powder is projected toward the substrate so that the aerodynamicforces bring the powder particles as close as possible to the substrate,where electrostatic forces predominate and cause the particles to beattracted to and deposited on the grounded substrate. The coatedsubstrate is then placed in an oven or furnace where the individualpowder particles melt, flow and form a continuous film on the substrate.

Several process aspects are involved in ESP, among which are powdercharging, powder transport, adhesion mechanisms, self-limitation, backionization and Faraday cage effect.

During spraying the charged powder particles tend to travel along thelines of the electric field. As the thickness of the deposited powderlayer grows, the voltage on the surface of the powder will increase sothat oncoming charged particles tend to deposit onto regions of lowerthickness which have lower surface voltage. The thickness of the powderlayer thus tends to grow uniformly, at least on flat surfaces, and isthus inherently self-limiting or self-regulating.

However, as each successive batch of oncoming charged particles and freeions generated by the high voltage corona discharge approach the powderlayer already deposited, the point is reached where the charge on thelayer exceeds its dielectric strength and back ionization occurs. Atthis point any oncoming powder is rejected and loosely adhering powderon the surface falls off.

Also charged particles cannot readily penetrate into corners, recessesand other hard to reach zones of a complicated substrate, for example, atwo-sided can body, because the electric field lines cannot penetrateinto them due to the shielding or screening effect of the surroundingconductive body. This is called the Faraday cage effect which statesthat any empty space (free of charge) enclosed within a conductor isfree from any field. As a rule of thumb, an electric field will notpenetrate in such a cavity beyond a depth equal to the radius of thecavity opening. The electric field lines will concentrate on theshoulders (or rims) of the cavity, typically the most forward edgefacing the spray gun.

Some imperfections in the final coating may be the direct result ofdefects in the powder layer (e.g. from back ionization and Faraday cageeffects) prior to fusion of the powder layer.

The self-limiting aspect of ESP has significant implications. Forexample, unskilled operators are able to spray at least flat substrateswith only brief instruction and training since it is virtuallyimpossible to create runs, drips or sags which are characteristic ofspray applied liquid finishes. Further it makes possible the relativelyeasy and practical design of automatic spray installations. For example,multiple electrostatic spray guns may be mounted on reciprocators andpositioned in staggered opposition to each other in an enclosed spraybooth. Parts to be coated are then moved between the two banks of sprayguns where a uniform coating of powder is applied. And since the appliedlayer is self-limiting, sufficient powder can be charged and applied tobe sure there are no unduly thin or uncoated areas. Overspray powder iscaptured in the reclaim system and reused.

Compared to flame spraying and fluidized bed coating, some majoradvantages of ESP are that generally thinner films on the order of 75microns (3 mils) or less can be consistently applied, smaller quantitiesof powder are used, and more intricately shaped substrates can be moreuniformly coated. Consequently ESP has become a firmly establishedtechnology in which coatings, both thick and thin, can be consistentlyand uniformly applied on substrates, both flat-surfaced articles as wellas some complicated or intricate shapes. ESP has proven suitable forapplying a wide variety of such insulator or dielectric coatings asplasticized PVC, nylon, cellulosic, epoxy, polyurethane, polyester,acrylic and hybrid resin powders on a wide range of conductivesubstrates, especially metallic articles, such as can bodies, wiregoods, pipe, tool housings, fire extinguisher bodies, householdappliances, floor polishing machinery, sewing machine parts, hospitalbeds, trailer hitches, parts and accessories for automobile, motorcycleand bicycle, furniture for lawn, garden, office and home, and structuralsections and facade elements.

The powder coating of wood has been much discussed in the trade and inthe literature but very little has been said as to how it may beaccomplished. As Douglas S. Richart stated in his article published inthe April, 1996 issue of POWDER COATINGS, the coating of wood with a lowtemperature cure powder is next to impossible because the coating mustbe cured at a temperature below 200° F. and the resin must have a flowtemperature of about 10 to 20 degrees lower than that. Such a resintends to block during storage at normal temperatures. Richart goes on tosay that the curing agent must be sufficiently reactive that the powderwill cure in a reasonable time at such low temperatures. But that leadsto a possible thermosetting of the resin in the extruder. He speaks ofelectrostatically spraying a powder having a blocked isocyanate ontowood, heating the coating in infra-red and other type ovens to form asmooth coating and curing the smooth coating with ultra-violetradiation.

In its technical bulletins, Boise Cascade shows the use of hand-heldelectrostatic spray guns in coating its electrically conducting particleboard.

The resin powders are typically produced by melt-mixing techniques, forexample, by homogeneously mixing such basic ingredients as the resin,hardening agent, flow agent, pigments, fillers and other additives;feeding the mixture to a high shear mixer, for example, an extruderwhere the meltable ingredients fuse at a temperature of about 90° to150° C. (194° to 310° F.), depending on the resin type, and thenonmeltable ingredients are intermixed with the melted material, forexample, by the augers of the extruder; feeding the material exiting themixer usually in the form of a continuous ribbon or strand into asystem, for example, of distributing rolls and conveyor where theextrudate is water cooled to room temperature; crushing the thoroughlycooled extrudate into small chips; and then granulating and grinding thechips into small particles which are then generally sifted through, forexample, a 140 micron sieve to obtain a homogeneous powder. The largerparticles are typically reclaimed and recycled back through the grinder.

A major disadvantage, however, of ESP using a corona discharge gun isthat a high voltage field is set up between the gun and the part to becoated. Complicated substrates or parts having deep angles, recesses,etc., are very difficult to coat principally because of theaforementioned Faraday cage effect. Indentations, inner edges, reentrantcorners, struts and surfaces overlapped by, for example, bonded-on partsare typical of the problematic zones needing coating. Due to thisgeneral inability to effectively coat such zones, overspray powdercaptured in the reclaim system has become a major problem in theindustry which has led to the development of various solutions includingnew and improved corona discharge gun designs, as well as more efficientreclaim systems.

Faraday cage or electrostatic screening effects alone cannot explain alldifficulties encountered in coating problematic areas. For example,aerodynamic effects have been found to be at least partially responsiblein some situations, especially where constricted areas exist. The air orgas propelled powder, often deposited sparingly in these zonesinitially, is thus repeatedly blown away by aerodynamic (e.g. venturi)effects, thus satisfactory coating of these areas is often almostimpossible.

It has been found that the difficulties mentioned can only be overcomeby taking into account electrostatic and aerodynamic laws. Only veryprecise matching of all electrostatic and aerodynamic parameters canresult in any fundamental improvement in the corona ESP system andequipment, principally due to the intrinsic electrostatic field betweenthe corona and the article to be coated.

Another approach has been the use of a triboelectric gun in place of thecorona discharge gun in the aforementioned ESP system. With this typetriboelectric gun, as exemplified in U.S. Pat. Nos. 3,903,321 and4,071,192, powder charging occurs by the frictional contact of the airtransported powder particles with the interior tubular surfaces of thegun, relying on the phenomenon of electrical charging which occurs whentwo dielectric or insulating materials (i.e. the powder and the gunsurfaces) are caused to be rubbed against each other. In theory, theeffect achievable is dependent on the dielectric constant of thematerials used. As reported by R. P. Lehman in a 1988 article entitled"Optimized Powder Coating of Critical Objects", an exemplary list ofsuch materials, in descending order of their relative dielectricconstants from the positive end to the negative end, is as follows:

    ______________________________________                                        Dielectrical Series                                                                       Positive end                                                      ______________________________________                                        24            Polyethylene oxide                                              23            Polyurethane                                                    22            Plexiglass                                                      21            Epoxy resin                                                     20            Polyvinyl acetate                                               19            Glass                                                           18            Urea/formaldehyde                                               17            Wool                                                            16            Polyamide (Nylon)                                               15            Polyvinyl alcohol                                               14            Cellulose                                                       13            Metals                                                          12            Rubber                                                          11            Cellulose acetate                                               10            Polyester resins                                                9             Polystyrene                                                     8             Anthracene                                                      7             Silicone                                                        6             Nitrocellulose                                                  5             Polyacrylonitrile                                               4             PVC                                                             3             Polypropylene                                                   2             Polyethylene                                                    1             Polytetrafluoroethylene (PTFE)                                                Negative end                                                    ______________________________________                                    

To achieve maximum chargeability it is believed that the two materialsshould differ considerably in electronegativity. It is easy to see fromthe above list why pure PTFE has generally become the material of choicein the industry for the gun's rubbing material. It can also seen, forexample, that epoxy powder is much more suitable than polyester powderfor charging in a gun using PTFE. However, other materials, such asnylon, have been successfully used as the rubbing material whenever thecoating powder chosen, such as polypropylene, is too close to PTFE inthe dielectric series.

The exemplary list of 24 materials above are the pure resins ormaterials. However, coating powders normally contain not only resinsbut, for example, up to 50% by weight of fillers, pigments and otheradditives which can influence chargeability. All powder paint systems donot lend themselves easily to tribocharging. Due to low polarity,several resin systems suffer from insufficient chargeability, forexample, carboxy functional polyesters. Attempts have been made to solvethis problem through addition of special additives, e.g. amines, toobtain the required chargeability by stabilizing positive charges. See,for example, European Pat. No. 371,528. However, such additives havedisadvantages, for example, they may separate from the powder in thereclaim recycling process, yielding a negative influence on the powdercoating properties, etc. Amines, for example, are also catalysts forunwanted chemical reactions (e.g. in the pre-mix extruder) and canadversely affect the cure behavior of the powder paint.

One explanation of the triboelectric coating system suggests thatelectrons are separated from the powder particles which becomepositively charged upon exiting the PTFE gun and are thus attracted tothe substrate which is earthed or grounded. The tribo-gun also usuallyhas an electrode (grounded) to remove the equal and opposite chargewhich builds up on the gun barrel. See, for example, the Journal ofElectrostatics, 16, 1985, pages 277-286, and SME Technical Paper (FC89-626) by J. Dailidas, Oct. 16-19, 1989, 12 pages, for further detailsregarding the principles of triboelectronics.

By utilizing a tribo-gun the high voltage and the strong electrostaticfield between the spray gun and the article (characteristic of thecorona gun) is dispensed with and the Faraday cage and back ionizationeffects are reduced, hence penetration of the powder into problematicareas of complicated shapes, as aforementioned, is greatly facilitated.This replacement of the corona gun with a tribo-gun reduces thepenetration problem generally to one of controlling the aerodynamicconditions. Some of the remaining variables influencing efficiency ofthe triboelectric coating systems are contact time, conductivity,temperature, humidity and particle size.

It is know that variations in average particle size (Mv) and particlesize distribution (Mx) have important implications on the operation ofelectrostatic powder coating systems. In general most electrostaticpowders have a Mx from about 150 to 200 microns down to sub-micronlevels, through the Mv falls in the range of about 20-50 microns. It isnow common practice to remove the coarse particles, for example, bysieving as they are known to have an adverse effect on the finalcoating, e.g. reduced smoothness. It is also generally known that finesof less than about 10 microns are detrimental to efficient coating andmay be removed, for example, by air classification.

For more particulars on various aspects of the background prior art, seePolym.-Plast. Technol. Eng., 7(2), 1976, pages 119-220; Kirk-Othmer:Encyclopedia of Chemical Technology, vol. 19, 3rd Ed., 1982, pages 1-7;Paint and Resin, October 1989, pages 8 and 10; Products FinishingMagazine, January 1990, pages 1-8; "The Particle Size Distribution ofPowder Coating", by K. Swamborn presented at May 15-16, 1990, seminar,24 pages, Hosokawa MicroPul; Proceedings of the Eighteenth Water-borne,Higher-Solids, and Powder Coatings Symposium, Feb. 6-8, 1991, "PowderCoatings--A Brief Review of the Technology", by D. Richart, pages191-211; Particle News, Issue #2, 2 pages, British Rema Manuf. Co.,Ltd., and U.S. Pat. Nos. 3,822,240; 4,027,066; 4,056,653, 4,109,027;4,113,681; 4,154,871 and 4,172,776.

A two component powder coating system and process for triboelectriccoating of wood is described in assignee's copending applicationentitled "Two Component Powder Coating System and Method for CoatingWood Therewith" U.S. patent application Ser. No. 08/643,694 filed May 6,1996, now abandoned.

Examples of further prior art dealing with electrostatic thermosettingcoating powders are illustrated by samples J, K and N in Table 7 below.

SUMMARY OF THE INVENTION

The principal object of the present invention is to electrostaticallyspray coat plastic powder via triboelectric coating procedures onto woodsubstrates in an efficient manner thereby minimizing overspray andreclamation efforts. It is a related object of this invention to providea low temperature method for producing a continuous protective ordecorative coating on wood having a minimum number of pinholes.

Briefly, these and other objectives of this invention are achieved bylimiting the quantity of coarse and fine particles in the dielectriccoating powder so that a mass thereof has an overall average particlesize (Mv) of between about 30 and 45 microns and, most critically, has aparticle size distribution (Mx) of:

0-5% larger than 88 microns (=95-100%<88μ),

85-95% larger than 15.56 microns (=5-15%<15.56μ) and

94-100% larger than 11 microns (=0-6%<11μ).

Preferably the coating powder mass has an Mx, as above defined, with 0%larger than 88 microns (=100%<88μ) and an Mv equal to about 30-40,preferably about 35-40, microns. More preferably the powder mass has anMx of:

0% larger than 88 microns (=100%<88μ),

85-90% larger than 15.56 microns (=10-15%<15.56μ) and

94-96% larger than 11 microns (=4-6%<11μ)

and an Mv equal to about 35-36 microns. Most preferably the powder masshas an Mx of:

0% larger than 88 microns (=100%<88μ),

about 88.5% larger than 15.56 microns (=11.5%<15.56μ) and

about 95.7% larger than 11 microns (=4.3%<11μ)

and an Mv of about 35.9. Preferably the powder mass is composed of athermosetting resin base formulation.

In accordance with a further aspect of the invention the improvementalso extends to an ESP process for coating substrates wherein the abovedefined mass of insulating or dielectric powder is delivered or chargedto a triboelectric spray gun.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in graph form Transfer Efficiency by plotting percenttransfer efficiency versus wt. % of particles smaller than 15.56 micronsdelivered to gun.

FIG. 2 illustrates in graph form Gun Current by plotting gun current inmicroamps versus wt. % of particles smaller than 15.56 microns deliveredto gun.

FIG. 3 illustrates in graph form Charge to Mass by plotting the powdercharge on the mass of powder sprayed (C/g) versus wt. % of particlessmaller than 15.56 microns delivered to gun.

FIG. 4 illustrates in graph form a commercial run of multiple samples of10-7272 powder by plotting Mean Value versus Number of Parts Coated.

DETAILED DESCRIPTION OF THE INVENTION

A more complete understanding of the invention will be apparent from thedetailed description to follow of the preferred embodiments of theelectrostatic coating powder and triboelectric coating process utilizingsuch powder for coating wood substrates. The critical role of theparticle size variable on tribocharging systems has been investigated indepth in an effort to reduce charging and coating variabilities.

The coating powders of the invention have thus been specificallydesigned and formulated for tribocharging, which is a known modificationof conventional ESP coating systems and procedures as set forth in thepreceding background information. The applicable coating powders underthis invention may be any of the materials aforementioned with respectto the corona discharge technique, preferably thermoset-based powdersincluding epoxies, acrylics, polyesters, polyurethanes and hybridsthereof.

The coating powder of this invention may be used on metallic substratesbut its particular utility for coating woods makes it highly appealingas a commercially viable alternative to processes such as laminatingfilms and coating liquids that have been almost universally used in thepast. For the purposes of this invention, wood is defined as anylignocellulosic material whether it comes from trees or other plants andwhether it be in its natural forms or its fibers have been separated,felted, and compressed to form hardboard, medium density fiberboard, orthe like. Particle board, whether standard or treated to enhance itselectrical conductivity, and oriented strand board are also within thedefinition of wood for this invention. Wood having a moisture content offrom 3 to 10% by weight is suitable for the purposes of this invention.

As aforementioned, tribocharging offers well documented advantages overcorona discharge in that tribo-guns require no high voltage supply,obtain better Faraday cage penetration, create less back ionization,achieve a wider range of minimum and especially maximum coatingthicknesses, produce smoother and more continuous films, and achievemore consistent overall performance, especially in coating articles ofcomplex configuration. The improvement is centered upon a modificationof particle size which has led to unexpected results in overalltriboelectric coating efficiency. The Mv and Mx were measured throughoutthis text by a Leeds & Northrop MICROTRAC® analyzer in the dry mode.

In an embodiment of the invention a thermosetting starting powder,CORVEL® 10-7199, was prepared by the aforementioned conventionaltechnique of melt compounding and grinding a cooled extrudate. (CORVELis a registered trademark of the assignee.) The resulting particle sizedistribution (Mx) of a Sample E of 10-7199 as set forth in Table 1 belowis typical of powders which are commercially available for electrostaticspray application via, for example, corona discharge ESP equipment.Based upon the most up-to-date knowledge of tribocharging applicationsand trade literature, it was first thought that a coarser distributionwould improve 10-7199. See Sample F, Table 2 below. While Sample Fprovided some improvement, it was generally unacceptable because thecoarser distribution caused large swings in performance as reclaimpowder was recycled and mixed with virgin powder. Also unacceptableamounts of overspray powder tended to increase or build up in thecoating system. Sample E was then further modified by air classification(A/C) resulting in an overall narrower distribution. See Samples A, B, Cand D of Table. The resulting Sample A-D powders showed a markedimprovement in Transfer Efficiency (see Table 3 below), Gun Current (seeTable 4 below) and Charge to Mass ratio (see Table 5 below), which threemeasures are used to most effectively evaluate tribochargeability.

Quite surprisingly, it was observed than an optimum point in the Mxrange beyond which further classification yielded no further benefits.See FIGS. 1, 2 and 3. The optimum is significant not only from a powderperformance standpoint, it also reduces the potential economic penaltiesderived from reworking removed fines. That is to say fines removed byair classification, to produce the desired Mx, are typically reextrudedand reground to reduce waste. Although this improves the efficiency ofthe classification, it is an added step with an associated cost. Thus itis essential to minimize the amount of fines removed to reach thedesired performance characteristics.

FIG. 1 graphically depicts Transfer Efficiency by plotting percenttransfer efficiency, E_(t) (on the ordinate) versus the weight percentof particles less than 15.56 microns delivered to the tribogun (on theabscissa). The E_(t) value for a given powder sample or lot is derivedby obtaining the weight (D) of the sprayed powder adhering to asubstrate, dividing D by the total weight (W) of the powder delivered bythe gun times one hundred. Thus the formula for calculating % E_(t)=D/W×100. The plotted values for E_(t) are typically an average of threereadings per sample.

FIG. 2 graphically depicts Gun Current by plotting the gun current (onthe ordinate) versus the weight percent of particles less than 15.56microns delivered to the tribo-gun. The gun current value for a givenpowder sample is the current produced in the tribo-gun which istypically measured by a LED microameter in the ground circuit for thegun. Typically after starting the powder loaded gun the amperagereadings are repeated three times at about one minute intervals.

FIG. 3 graphically depicts Charge to Mass by plotting the ratio of theelectrical charge on the mass of powder being tribo-charged (on theordinate) versus the weight percent of particles less than 15.56 micronsdelivered by the gun. The ratio of electrical charge on the powder mass(M_(c)) is defined as a unit of electrical charge expressed in Coulombs(C) divided by the weight (g) in grams of the powder delivered to thegun; thus M_(c) =C/g. The charge to mass value for a given powder sampleis typically obtained by meter readings using a Wolfson electrostaticpowder coating test kit. The plotted values typically are an average ofthree readings for a given sample.

Further, air classified powder so produced (CORVEL 10-7272) yieldedimproved, consistent performance in a commercial triboelectric coatingsystem as demonstrated in FIG. 4, which graphically depicts Mean Value(on the ordinate) versus Number of Parts Coated (in thousands). MeanValue represents the average particle size in microns of the 10-7272powder delivered to the gun by way of a conventional fluidized bedsystem, including any recycled reclaim. Several different preparations(same composition) of samples or lots of 10-7272 were tested and plottedin FIG. 4. All initial samples were virgin 10-7272 within thedistribution range:

0-5% larger than 88 microns, 88-95% larger than 15.56 microns and94-100% larger than 11 microns, and an Mv of 30-40 microns.

                                      TABLE 1                                     __________________________________________________________________________    PARTICLE SIZE                                                                          SAMPLE                                                                                        E      G          L                                                           (CORVEL                                                                              (CORVEL    (CORVEL)                           % RETAINED @                                                                           A*  B*  C*  D*  10-7199 stock)                                                                       10-7223 stock)                                                                       H** 10-7068 stock)                                                                       M***                        __________________________________________________________________________    124.45μ                                                                             0.0 0.0 0.0 0.0 0.0    0.0    0.0 0.0    0.0                          88.00μ                                                                             0.0 0.0 0.0 0.0 5.4    2.59   2.37                                                                              0.0    0.58                         62.23μ                                                                             11.4                                                                              8.0 9.3 5.1 11.6   17.62  22.22                                                                             19.7   18.78                        44.00μ                                                                             37.1                                                                              29.5                                                                              30.1                                                                              22.9                                                                              22.6   34.38  44.68                                                                             40.07  39.30                        31.11μ                                                                             65.8                                                                              56.5                                                                              54.3                                                                              45.0                                                                              41.4   49.47  63.75                                                                             55.44  58.57                        22.00μ                                                                             82.9                                                                              75.7                                                                              72.6                                                                              64.4                                                                              55.0   61.79  80.17                                                                             71.90  76.27                        15.56μ                                                                             91.9                                                                              88.5                                                                              85.7                                                                              78.7                                                                              66.7   72.59  91.57                                                                             81.12  89.01                        11.00μ                                                                             97.0                                                                              95.7                                                                              94.3                                                                              89.9                                                                              76.8   82.21  98.11                                                                             90.58  97.18                        7.78μ                                                                              98.8                                                                              98.4                                                                              98.1                                                                              95.8                                                                              84.1   88.23  100.00                                                                            96.49  99.49                        5.50μ                                                                              100.0                                                                             100.0                                                                             100.0                                                                             96.7                                                                              89.3   92.87  100.00                                                                            100.00 100.00                       3.89μ                                                                              100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             94.3   96.64  100.00                                                                            100.00 100.00                       2.75μ                                                                              100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             97.9   98.70  100.00                                                                            100.00 100.00                       1.94μ                                                                              100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                             100.0  100.00 100.00                                                                            100.00 100.00                      Average Size, Mv                                                                       39.6μ                                                                          35.9μ                                                                          35.5μ                                                                          31.2μ                                                                          31.4μ                                                                             35.80μ                                                                            43.41μ                                                                         38.95μ                                                                            40.24μ                   DISTRIBUTION                                                                  WIDTH                                                                         (Analyzer Channels                                                                     7   7   7   8   11     12     7   8      8                           Reporting)                                                                    % <15.56μ                                                                           8.07                                                                              11.50                                                                             14.30                                                                             21.30                                                                             33.30  27.41  8.43                                                                              18.88  10.99                       __________________________________________________________________________     *CORVEL 107272 (A/C 107199)                                                   **A/C CORVEL 107223                                                           ***A/C CORVEL 107068                                                     

                  TABLE 2                                                         ______________________________________                                        PARTICLE SIZE                                                                 % RETAINED @        SAMPLE F*                                                 ______________________________________                                        248.90μ          1.2                                                       176.00μ          4.5                                                       124.45μ          13.1                                                      88.00μ           25.1                                                      62.23μ           42.9                                                      44.00μ           59.5                                                      31.11μ           69.4                                                      22.00μ           78.9                                                      15.56μ           87.2                                                      11.00μ           91.1                                                      7.78μ            94.2                                                      5.50μ            96.7                                                      3.89μ            98.5                                                      2.75μ            100.0                                                     1.9μ             100.00                                                    AVERAGE SIZE, Mv    66.4                                                      DISTRIBUTION WIDTH  13                                                        (Analyzer Channels Reporting)                                                 % < 15.56μ       12.8                                                      ______________________________________                                         *coarse CORVEL 107199                                                    

                  TABLE 3                                                         ______________________________________                                        TRANSFER EFFICIENCY                                                           ______________________________________                                        SAMPLE   A         B      C      D    E                                       ______________________________________                                        (%)      32.0      32.0   28.3   23.7 18.9                                    ______________________________________                                                 G         H      I      L    M                                       ______________________________________                                                 24.71     31.55  38.39  24.46                                                                              27.55                                   ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        GUN CURRENT                                                                   ______________________________________                                        SAMPLE    A         B      C      D    E                                      ______________________________________                                        (Microamps)                                                                             1.0       3.25   2.25   2.0  0.5                                    ______________________________________                                                  G         H      I      L    M                                      ______________________________________                                                  0.5       2.00   2.00   1.00 2.25                                   ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        CHARGE TO MASS                                                                ______________________________________                                        SAMPLE      A        B      C      D    E                                     ______________________________________                                        (x 10.sup.-7                                                                              10.2     11.2   12.0   8.3  2.5                                   Coulombs/gram)                                                                ______________________________________                                                    G        H      I      L    M                                     ______________________________________                                                    8.30     10.9   7.79   6.3  12.2                                  ______________________________________                                    

In another embodiment of the invention the Mx of another commerciallyavailable stock or starting thermosetting powder, CORVEL 10-7223 (whichwas similarly prepared as the 10-7199 stock), is set forth as Sample Gin Table 1. And similarly to coarse Sample F in Table 2, a coarse SampleI of 10-7223 was prepared; see Table 6 below. While showing someimprovement, Sample I was generally unacceptable for similar reasonsstated regarding Sample F. Stock Sample G was then further modified toform Sample H, Table 1, by air classification resulting in an overallnarrower Mx. The resulting Sample H powder, similarly to Samples A, Band C of 10-7272, showed a marked improvement in Transfer Efficiency,Gun Current and Charge to Mass. See Tables 3, 4 and 5 respectively.

                  TABLE 6                                                         ______________________________________                                        PARTICLE SIZE                                                                 % RETAINED @        SAMPLE I*                                                 ______________________________________                                        248.90μ          0.0                                                       176.00μ          0.0                                                       124.45μ          1.67                                                      88.00μ           13.22                                                     62.23μ           36.72                                                     44.00μ           63.21                                                     31.11μ           83.15                                                     22.00μ           93.90                                                     15.56μ           98.40                                                     11.00μ           100.00                                                    7.78μ            100.00                                                    5.50μ            100.00                                                    3.89μ            100.00                                                    2.75μ            100.00                                                    1.94μ            100.00                                                    AVERAGE SIZE, Mv    57.04                                                     DISTRIBUTION WIDTH  8                                                         (Analyzer Channel Reporting)                                                  % >15.56            98.40                                                     % >11.00            100.00                                                    % >88.00            13.22                                                     ______________________________________                                         *Coarse CORVEL 107223                                                    

In yet another embodiment of the invention the Mx of anothercommercially available stock or starting thermosetting powder, CORVEL10-7068, which was similarly prepared as the 10-7199 and 10-7223 stock,is set forth as Sample L in Table 1. Stock Sample L was then furthermodified to form Sample M, Table 1, by air classification resulting inan overall narrower Mx. The resulting Sample M powder, similarly toSamples A, B, C and H, showed a marked improvement in TransferEfficiency, Gun Current and Charge to Mass. See Tables 3, 4 and 5,respectively.

Consequently, based on the specific embodiments of the invention;namely, samples A, B, C, H and M, the novel mass of electricallychargeable plastic powder which is tribocharged broadly has a particlesize distribution (Mx) of (all percents defined in weight percent):

0% -5% larger than 88 microns (=95-100%<88μ),

85%-95% larger than 15.56 microns (=5-15%<15.56μ) and

94%-100% larger than 11 microns (=0-6%<11μ),

and an average particle size (Mv) of between about 30 and about 45microns.

Preferably the dielectric coating powder mass according to the invention(Samples A, B and C) has an Mx as above defined with 0% larger than 88microns (=100%<88μ) and an Mv of about 30-about 40 microns, preferablyabout 35-40 microns.

More preferably the dielectric powder mass of the invention (Samples Band C) has an Mx of:

0% larger than 88 microns,

85%-90% larger than 15.56 microns (=10-15%<15.56μ) and

94%-96% larger than 11 microns (=4-6%<11μ),

and an Mv of about 35-about 36 microns.

Most preferably the dielectric powder mass has an Mx as shown in SampleB of:

0% larger than 88 microns,

about 88.5% larger than 15.56 microns

(=about 11.5%<15.56μ) and

about 95.7% larger than 11 microns (=about 4.3%<11μ),

and an Mv of about 35.9.

Preferably the powder mass according to the invention is a formulationbased on a thermosetting resin.

For comparative purposes, Samples J, K and N of prior art thermosettingpaint powders are set forth in Table 7 below. The particle size ofSamples J and K are too coarse, i.e. the percent retained greater than88 microns is too high, and would be expected to perform in much thesame unacceptable fashion as preceding coarse Samples F and I. Theparticle size of Sample N is too low in the percent retained at both15.56 and 11 microns; thus Sample N would be expected to perform in muchthe same unacceptable manner as preceding similar Sample D.

                  TABLE 7                                                         ______________________________________                                        (PRIOR ART)                                                                   PARTICLE SIZE                                                                 SAMPLE                                                                        % RETAINED @      J*       K**     N***                                       ______________________________________                                        124.45μ        0.0      4.64    0.0                                        88.00μ         8.44     21.12   0.0                                        62.23μ         27.48    46.28   6.42                                       44.00μ         51.27    68.74   26.80                                      31.11μ         70.21    83.27   46.05                                      22.00μ         82.05    91.53   64.19                                      15.56μ         90.87    96.46   77.83                                      11.00μ         95.97    99.02   87.47                                      7.78μ          98.45    100.00  92.19                                      5.50μ          100.00   100.00  94.60                                      3.89μ          100.00   100.00  100.00                                     2.75μ          100.00   100.00  100.00                                     1.94μ          100.00   100.00  100.00                                     AVERAGE SIZE, Mv  48.11μ                                                                              63.36μ                                                                             31.91μ                                  DISTRIBUTION WIDTH                                                                              9        8       9                                          (Analyzer Channels Reporting)                                                 % <15.56μ      9.13     3.54    22.17                                      ______________________________________                                         *Becker ER7493 SG                                                             ** Tiger White Base                                                           *** Lilly 907 BS                                                         

The triboelectric coating process of the present invention isparticularly effective in situations such as when the wooden substrateis profiled. The grooves and ridges present a particular problem forelectrostatic coating processes because of the Faraday effect. Becausethe electrical charge generated by friction as the powder flows alongthe TEFLON plastic surfaces inside the gun are relatively small incomparison with the charge picked up as the powder flows through acorona-discharge cloud. Wooden cabinet doors are examples of a profiledwooden substrate as are the doorskins represented by the drawings inU.S. Pat. No. 5,489,460, which further illustrates the type of woodenpanels that are particularly susceptible to powder coating by the methodof this invention. The grooves and sharp edges of such panels arecovered very well on a flat line coating apparatus with nozzles arrayedto direct a portion of the powder against them.

Such panels as well as flat-surfaced panels such as those used to makeping-pong tables are particularly well coated by triboelectric guns on aflat line conveyor having electrically conductive bands around thecircumference of the conveyor belt. Apparatus for such coating isdisclosed in a series of patents assigned to the Nordson Corporation.These are U.S. Pat. Nos. 4,498,913; 4,590,884; 4,723,505; 4,871,380;4,910,047; and 5,018,909.

A suitable flat line powder coating apparatus comprises such a conveyorextending through a powder coating booth, wherein a wooden articlesupported and moved by the conveyor belt is coated triboelectrically bya plurality of guns situated adjacent one another and in one or moretiers. The powder is forced into the guns under about 40 psi pressureand air at about 20 psi is passed into the powder conduits just beforethe powder passes into the nozzles. The article bearing the powder isthen conveyed through a curing oven having several heating zones, someof which are heated by IR lamps, others by heat convection, and stillothers by a combination of those two. The coating and the curing linespeeds may be the same or different, depending on the length of thecuring oven. The line speed through the powder application booth may befrom about 5 to about 150 feet per minute but preferably from about 20to about 100 feet per minute. The line speed through the curing oven, onthe other hand, may be from about 5 to about 20 feet per minute,depending on the oven temperature, the length of the oven, and theparticular coating powder used. The curing temperature may range fromabout 180° up to but not including the decomposition temperature of thepowder. It is preferred to maintain the cure temperature within therange of from about 190° to about 300° F. and still more preferred tokeep the cure temperature at from about 250° to about 300° F. When acrystalline epoxy resin is used, a cure temperature of about 180° F. isparticularly suitable. It is preferred that the coating and curing linespeeds be adjusted to the oven length so that they are balanced.

Preheating of the panel before the coating step is preferred in someinstances, e.g., to help the powder reach its flow temperature in thefirst zone of the oven and it also minimizes outgassing during cure. Theoven may be several heating zones, both IR and convection type and alsoa combination of the two. The TRIAB Speedoven sold by ThermalInnovations Corporation is suitable for the purposes of this invention.The film thickness of the cured coating is at least about 1 mils and itmay be as much as about 8 mils or even higher if there is a practicalneed for such. Film thicknesses of from about 4 to about 6 mils areachieved regularly by the method of this invention, even at coating linespeeds of about 100 feet per minute.

In a further embodiment lab tests run to determine the wood coatingcharacteristics of powders prepared on a common basis having a particlesize distribution within the invention and falling outside the scope ofthe invention, i.e. the only variable resides in the particle sizedistribution. Identical size pieces of particle board 51/4×23/4 weretriboelectrically coated with a hand-held gun using 10 grams of thepowders sprayed out completely. The thickness of the coating was thenmeasured after a 300° F.--10 minute cure cycle. The results of the fivepoints measured on each piece in mils were as follows:

                  TABLE 8                                                         ______________________________________                                                    Top  Bottom        Bottom                                                                              Top                                                  Right                                                                              Right   M     Left  Left Av.                                 ______________________________________                                        Sample O (CORVEL 265-                                                                       5      1       1.4 2     2.25 2.33                              148-2 *Brandy Red                                                             Sample P (CORVEL 23-                                                                        4.1    4.8     6.0 6.2   3.9  5.00                              9296 *Brandy Red                                                              ______________________________________                                         *Brandy Red  0.4 parts/100 parts resin Chromaphtal Red 3B (CibaGeigy) and     137011 Red Violet (Hoescht).                                             

Sample O powder had a particle size distribution outside the inventionand Sample P had a particle size distribution within the invention. Itcan be seen from Table 8 that the Sample P (invention) powder provided athicker coating on the particle board test panels.

With this description of the invention in detail, those skilled in theart will appreciate that various modifications may be made to theinvention without departing from the spirit thereof. Therefore, it isnot intended that the scope of the invention be limited to the specificembodiments illustrated and described. Rather it is intended that thescope of the invention be determined by the appended claims and theirequivalents.

We claim:
 1. A coated wood substrate having a uniform continuousprotective or decorative plastic powder coating in a thickness range ofabout 1 to 8 mils, said plastic coating having been applied as a powderto said substrate using a triboelectric spray gun and wherein saidpowder has a particle size distribution, Mx comprising all in percent byweight:95%-100% smaller than 88 microns, 5%-15% smaller than 15.56microns and 0%-6% smaller than 11 microns.
 2. A coated wood substrateaccording to claim 1 wherein said powder has an average particle size Mvof between about 30 and 40 microns and a Mx comprising:100% smaller than88 microns, 5%-15% smaller than 15.56 microns and 0%-6% smaller than 11microns.
 3. A coated wood substrate according to claim 1 wherein saidpowder has an average particle size Mv of between about 35 and 36, and aMx of comprising all in percent by weight:100% smaller than 88 microns,10%-15% smaller than 15.56 microns and 4%-6% smaller than 11 microns. 4.A coated wood substrate according to claim 1 wherein the particle has anaverage particle size Mv of about 35.9 and a Mx of:100% smaller than 88microns, about 11.5% smaller than 15.56 microns and about 4.3% smallerthan 11 microns.
 5. The plastic powder coated wood substrate of claim 1wherein the substrate comprises a nonuniform or raised panel structure.6. The plastic powder coated wood substrate of claim 1 wherein said woodsubstrate comprises a particle board.
 7. The plastic powder coated woodsubstrate of claim 1 wherein said wood substrate comprises orientedstrand board.
 8. The plastic powder coated wood substrate of claim 1wherein the powder is a thermosetting resin.
 9. The plastic powdercoated wood substrate of claim 1 wherein said wood substrate comprisesmedium density fiberboard.